1. Field of the Invention
The present invention relates to a gravity base offshore production platform structure and, more particularly, a production platform for use in an arctic or sub-arctic environment, including an encircling structure having a predetermined configuration, giving protection to the offshore structure in iceberg-infested waters. This structural configuration is capable of resisting the destructive forces of an impact produced by an iceberg.
More recently, the increased worldwide demand for hydrocarbons such as oil and natural gas, has necessitated the investigation and exploitation of many new regions throughout the world, both on land and offshore. One of the regions which appears to be extremely promising in its potential for finding hydrocarbon fuels is the offshore arctic and sub-arctic area in proximity to Canada and Greenland. However, this area calves numerous icebergs each year. The size and shape of these are dependent upon their glacial source, climatic and hydrographic conditions determinative of their survival or deterioration, and the routes and distances over which the icebergs travel. Some of these icebergs reach a very large magnitude and, when impacting against a structure, produce immense destructive forces.
The offshore production platform, especially the support section thereof which is submerged below the marine surface, is exposed to the impact of large icebergs. In addition to experiencing localized or massive failures in the structure thereof, it can to some extent slide along the marine floor, conceivably causing damage to expensive and difficultly replaceable equipment, wall casings and pipelines connected to the platform support structure. In order to counteract any substantial or massive foundation or support section failure of the offshore platform, the submerged support section of a self-protecting type of platform should be impressive in its mass and dimensions so that possible failure of structural and foundation components are limited to localized areas. While these localized areas may conceivably fail under the impact of large icebergs having extremely strong ice, this should occasion only a brief and temporary supension of platform operations when such an impact encountered reaches a maximum or extreme limit. The localized impact from very strong ice may cause flexural and shear failure which extends progressively into the structure, rupturing oil tanks. The global or overall energy of the impacting icebergs, if not absorbed by some local energy absorbing means, may cause the entire structure to slide or tilt, thereby resulting in great economic loss.
2. Discussion of the Prior Art
Various solutions to the problems encountered in protecting offshore structures from damage caused by iceberg impact in iceberg-infested waters have been suggested in the prior art. Thus, Pearce et al. U.S. Pat. No. 4,245,929 discloses an offshore structure able to withstand ice forces generated by impinging ice sheets or icebergs in which at least the lower portion of the support structure of the offshore platform includes upper and lower differently sloped conical portions with respect to the walls thereof forming an inclination relative to the horizontal so as to receive and deflect ice masses moving into contact with the platform support structure. The particular structural selection of the conical wall structure is designed to cause the ice to tilt upwardly upon impinging against the support structure and to fragment itself while sliding off the support structure. The type of conical wall structure proposed in Pearce et al. does not appear to be adequate to withstand the impact of extremely large icebergs encountered in arctic or subarctic waters and is primarily intended for the purpose of deflecting relatively thin ice sheets rather than large and massive icebergs.
Howard U.S. Pat. No. 3,766,737 discloses an offshore platform which is encompassed, at a radial distance from the platform, by a circumferentially movable ice trenching machine which will circulate about the platform so as to fragment and remove ice in a circular path at a rate approximately equal to the rate of movement of the ice sheet towards the protected structure. Also this type of protective arrangement for offshore platforms is only adapted to protect the platform from the pressures of ice sheets and does not appear to provide any significant protection against the large destructive forces generated through impact by a massive iceberg.
Challine et al. U.S. Pat. No. 4,142,819 discloses an offshore platform in which the platform is of the gravity-type including a base resting on the marine floor and with an annular shell affording rigidity in the upward direction, such as a circular wall and diaphragms extending about the base portion of the platform so as to provide reinforcement therefore. This type of reinforcing support structure for the base of the offshore platform does not appear to be designed to withstand the impact of icebergs, particularly any relatively large and massive icebergs normally encountered in arctic and sub-arctic waters and thus would provide inadequate protection for the platform, thereby rendering it unsuitable for use in iceberg-infested waters.